Modern contraceptive methods have a surprisingly short history and are dominated by a few methods such as the oral contraceptive pill for women or physical barrier methods such as condoms for men. In spite of the lack of choice of methods, studies show that men aspire to play a greater role in the regulation of fertility. Some effort in the past has been focused on development of male contraceptives, which focused on hormonal and immunological control of spermatogenesis, but few studies have focused on developing a male contraceptive that will alter the activity of key genes that regulate spermatogenesis. This is the approach that we outline. It is a timely approach, as with the completion of the human genome project and those of model organisms, we have unprecedented knowledge of the genes and proteins that are likely to govern the development of sperm. In our preliminary studies, we have identified genes that encode male fertility factors in Drosophila, discovered homologs in mice and humans, and begun to explore mechanisms of male infertility by examining the phenotypes of sterile mutant flies and knockout (KO) mice. The objectives of this research are to identify and to characterize novel gene targets involved in the regulation of male fertility and to lay the ground work for the development of male contraceptives against those targets. We will screen a mouse ES gene trap cell library for male reproductive genes identified through our comparative genomics analysis, to generate mouse KO mutants disrupting the conserved male fertility factors, to characterize the roles of those key regulators during sperm production and maturation, and finally to provide a resource of male reproductive mutants for researchers in the contraceptive field, as well as in the reproductive field.